# std::atomic_ref<T>::exchange

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< cpp‎ | atomic‎ | atomic ref

value_type exchange( value_type desired, std::memory_order order = std::memory_order_seq_cst ) const noexcept; |  | (constexpr since C++26)

Atomically replaces the value of the referenced object with desired. The operation is a read-modify-write operation. Memory is affected according to the value of order.

This overload participates in overload resolution only if std::is_const_v<T> is false.

### Parameters

desired | - | value to assign
order | - | memory order constraints to enforce

### Return value

The value of the referenced object before the call.

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3508 (P3323R1) | C++20 | `exchange` was meaningless for const T | constrained to accept only non-const `T`

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